Biological signals, for example electrocardiogram (ECG) signals, may include substantial amounts of noise. For example, noise introduced by muscular activity, motion artifacts, etc, which usually appear as rapid, wavy deflections that render the ECG difficult to read, especially during the movement.
In this regard, U.S. Pat. No. 6,216,031B1 proposes an apparatus for enhancing signals in an ECG including artifacts, which apparatus comprises a mean value unit for evaluating the curve shape of a predetermined number of ECG signal beats from the beginning of a QRS complex to the end of a T wave, and using the outcome to establish a mean value beat, and it further comprises a subtracting unit for subtracting the ECG signal's mean value beat from an actual beat to obtain a residual signal, and a FIR filter unit for high-low-pass filtering the residual signal to provide a filtered signal to which the mean value beat is added in an adding unit.
The principle of U.S. Pat. No. 6,216,031B1 is to utilize the mean value unit to calculate the mean value beat based on averaging of successive ECG cycles and utilize the LP (low pass) and HP (high pass) filters to deal with residual signals to remove both muscle noises and baseline wander noises. Although this method enhances the signal-to-noise ratio (SNR) of the resulted ECG signal to some extent, it still has some unacceptable defects.
Furthermore, in U.S. Pat. No. 6,216,031B1, normal LP/HP filtering skill is used to remove the noises. For example, the residual signal is filtered by a low-pass filter to reduce muscle noise, and by a high-pass filter to reduce baseline wander. The cutoff frequencies of the filters are set to values that avoid unacceptable distortion of the remaining P waves and VPBs in the residual signal. Both filters are finite impulse response filters with the advantage that the delay of the filtered residual signal is constant and signal-independent. But unacceptable notches may also appear, particularly in the case of an abrupt ECG morphology change.